1. Field of the Invention
The present invention relates to a backlight driving technology for an LCD (Liquid Crystal Display) device, and more particularly, to a lamp driving apparatus for an LCD device which is capable of implementing a high contrast ratio that cannot be implemented by only an analog dimming method or burst dimming method.
2. Description of the Related Art
As IT (Information Technology) develops recently, a display device plays an important role as a medium for displaying visual information. In order to be spotlighted in the future, the display device has to meet the following conditions such as a low power consumption, a thin thickness, a light weight, a high picture quality and so on.
An LCD device is a typical display device of flat panel display devices and operates to display images by using an optical anisotropy of a liquid crystal. The LCD device has advantages in that it is thin, small, low in power consumption, high in picture quality and so on, accordingly being developed as a main product of the flat panel display devices for replacing a cathode-ray tube (CRT).
Generally, the LCD device serves to display desired images by supplying image information to pixels arranged in a matrix shape, respectively, and adjusting a light transparency of the pixels. Thus, the LCD device includes a liquid crystal panel on which each pixel which is a minimum unit for implementing images is arranged in an active matrix shape, and a driving unit for driving the liquid crystal panel. Further, because the LCD device cannot emit light by itself, the LCD device is provided with a backlight unit for supplying light thereto.
Generally, a contrast ratio (CR) is obtained by dividing a highest brightness value on a screen by a lowest brightness value. Thus, in order to increase the CR, the highest brightness value has to be larger, or the lowest brightness value has to be smaller.
The brightness on the screen for the LCD device is determined by a dimming of the backlight. FIG. 1 shows an analog dimming method in the related art. According to the analog dimming method, the brightness is determined by controlling a tube current (or a tube voltage) supplied to the backlight unit. For example, if the tube current supplied to the backlight unit is maximized, the brightness is maximized. On the contrary, if the tube current is minimized, the brightness is minimized.
The analog dimming method can be simply implemented, but has a narrow dimming range of 100˜50%. Also, in a low current state, one of both ends of a lamp to which a power is supplied from an outside has a relatively higher brightness, while the other thereof to which the power is not supplied from the outside has a relatively lower brightness, accordingly the brightness may be ununiformly implemented.
FIG. 2 shows a burst dimming method in the related art. According to the burst dimming method, the brightness is determined as wanted by controlling a duty ratio of the tube current (or the tube voltage) by controlling an input voltage through a pulse width modulation integrated circuit (PWM IC). For example, if the duty ratio of the tube current supplied to the backlight unit is maximized, the brightness is maximized. On the contrary, the duty ratio of the tube current is minimized, the brightness is minimized.
The burst dimming method has advantages in that the dimming range is wide as 100˜20% and the brightness can be uniformly implemented at both ends of the lamp.
In some cases, the analog dimming method can be used together with the burst dimming method. That is, the analog dimming method and the burst dimming method can be applied to one backlight unit in some cases.
However, the related LCD device is configured to control the brightness by using one or both of the analog dimming method and the burst dimming method. In such case, it is difficult to increase the contrast ratio due to characteristics of the lamp.